For the River Thames River, water-quality monitoring at Hampton has produced continuous records for nitrate for the last 140 years, the longest continuous record of water chemistry anywhere in the world. In this article, water quality for the Thames River is combined with land use and land management data for the past 140 years to show how nitrate concentrations in the river have changed with agricultural intensification in the Thames River catchment.

The modeling approach used combines an estimate of nitrate available for leaching due to land use and land management and an algorithm to route this leachable nitrate through to surface or ground waters.

Prior to agricultural intensification at the start of World War II, annual average inputs were around 50 kg/ha, and river concentrations were stable at 1 to 2 mg/l.

Postintensification data suggest an accumulation of 100 (±40) kt N/yr in the catchment, most of which is stored in the aquifer.

There is an immediate effect 50% of the free nitrate goes straight into Thames tributaries and the main river as surface run-off. But here is also a delayed effect in which 50% of the nitrate works its way through ground aquifers before washing out into the Thames.

This build up of reactive N species within the catchments means that restoration of surface nitrate concentrations typical of the preintensification period would require massive basin-wide changes in land use and management that would compromise food security and take decades to be effective. Policy solutions need to embrace long-term management strategies as an urgent priority.

Nitrate pollution in intensively farmed regions: What are the prospects for sustaining high-quality groundwater?, Nicholas J. K. Howden, Tim P. Burt, Fred Worrall, Simon Mathias, and Mick J. Whelan WATER RESOURCES RESEARCH, VOL. 47, W00L02, 13 PP., 2011

BBC article, Jonathan Amos